The present invention relates generally to extraction processes and more particularly to a decaffeination of coffee by a continuous extraction process. A caffeine-laden aqueous extract of green coffee is prepared from which caffeine is extracted by the use of carbon dioxide as a solvent. The invention lies in the use of specific equipment under closely controlled conditions of pressure and temperature to provide high yields and economical costs of production.
In recent years, consumer demand for decaffeinated coffee has been increasing. About 20% of the coffee sold in the United States is decaffeinated. Most of this coffee is now decaffeinated in processes which employ synthetic organic solvents such as methylene chloride or ethyl acetate, either by directly contacting the coffee beans with the extracting organic solvent or indirectly, by contact of coffee beans with solvent-decaffeinated coffee extract.
There is public concern that decaffeinated coffee may contain inadvertent traces of organic solvent which might be toxic or carcinogenic. Furthermore, organic solvents remove other coffee constituents along with caffeine to the detriment of the coffee flavor. There has thus arisen a need for a process which does not use organic solvent.
The use of pressurized gases as solvents has long been known. For example see M. Centnerszwer, Z. Physik. Chem. 46, 427 (1903); H. Buchner, Z. Physik. Chem. 54, 665 (1906); M. Centnerszwer, Z. Physik. Chem. 72, 431 (1910); D. B. Todd and J. C. Elgin, A.I. Ch. E. Journal 1, 20 (1955).
Carbon dioxide, a normal constituent of roasted coffee, is a preferred solvent for decaffeination. It is produced naturally during the roasting process, typically to the extent of 2 to 3% of coffee weight. It is non-toxic and will selectively remove caffeine from coffee beans or coffee extract. It may be used for decaffeination at elevated pressure although much more carbon dioxide is required than conventional organic solvents. Consequently, for economic operation, highly efficient decaffeination equipment is required.
U.S. Pat. No. 4,260,639 to Zosel discloses a slow process, not readily made continuous, for the carbon dioxide decaffeination of coffee from batches of moistened green coffee beans in a period of 5 to 30 hours. In that process each batch of green (unroasted) coffee beans is extracted in a high pressure vessel which must be charged and closed. The carbon dioxide must then be admitted and pumped to a high pressure before decaffeination can be effected. After decaffeination the pressure must be released slowly so that the beans do not fracture by expansion of the carbon dioxide inside the beans. The vessel is then opened to discharge the decaffeinated coffee. The cost and time required for loading, caffeine extraction and unloading and the cost of re-pressurizing the carbon dioxide after each batch make this process economically unattractive.
An indirect "water process" decaffeination method is described in British Patent No. 314,059 to Klapproth and U.S. Pat. No. 2,309,092 to Berry et al. However, they employ an organic solvent, generally a chlorinated solvent, to decaffeinate an aqueous solution of green coffee solubles. The aqueous solution is then used to extract caffeine from green coffee beans. The caffeine-laden solution is recycled to the solvent. If water were used instead of the aqueous coffee solution, most of the coffee flavor precursors which develop flavor and aroma on subsequent roasting would be lost with the water and a substantial part of the coffee weight would also be lost. Recycling of the solution results in the removal from the coffee of primarily only those components (principally caffeine) which are extracted by the organic solvent from the solution. The "water process" has also been modified by others to use solid adsorbents such as activated carbon instead of a chlorinated organic solvent to remove caffeine from the solution. This method has the disadvantage that some other coffee components are removed as well as caffeine, reducing yield and weakening the coffee flavor. See for example, European patent application 111,375 to Moolweer.
German patent application No. 2,638,383 to van der Stegen describes a continuous process which substitutes carbon dioxide for the chlorinated organic solvent in the "water process". The van der Stegen suggestion has the advantage that coffee beans can be extracted with a water solution at low or atmospheric pressure and only this solution need be handled at elevated pressure in contact with carbon dioxide. The van der Stegen idea has been open to public inspection since Mar. 3, 1977, yet there has been no known commercial application of the process.
Extraction requires the transfer of a solute (caffeine) from one fluid (green coffee extract) to a second fluid (pressurized carbon dioxide). Close contact of the fluids followed by their effective separation is essential. Generally a series of such contacts and separations arranged countercurrently is required for efficient extraction. I have found that the green coffee extract has a strong tendency to form a foam when intermixed with pressurized carbon dioxide. This foam makes caffeine extraction and separation of the carbon dioxide from the solution as suggested by van der Stegan impractical in equipment of conventional design.
Packed extraction columns, such as described in U.S. Pat. No. 4,348,422 to Zosel are unsuitable because there is considerable vertical (axial) mixing of the two fluids resulting in poor efficiency and increased cost of operation. This is particularly severe in systems employing supercritical gases such as carbon dioxide; its low viscosity (about one fifteenth that of water) increases eddy flow and thereby interferes with the countercurrent contacting required for efficient extraction.
Mechanically assisted columns such as "rotating disc" columns or "Scheibel" columns are also subject to axial mixing. The mechanical agitation also promotes formation of emulsion or foam which inhibits separation of the two fluids.
Sieve plate extraction columns are well known to effectively prevent backmixing. See C. J. King "Separation Processes" McGraw-Hill, New York 1980, page 765. Attempts to decaffeinate green coffee extract with carbon dioxide in a conventional sieve plate column were unsuccessful. Contact and almost complete separation of the two fluids between successive plates in the column are essential for proper operation. Conventional design does not achieve this. Supercritical carbon dioxide in passing upward through plate perforations and through a green coffee extract forms a persistent foam which prevents operation of the process.
Prior to the instant invention, there has not been an economical and efficient countercurrent extraction process for the decaffeination of coffee in a continuous process employing supercritical carbon dioxide and no organic solvents.